This invention relates to oil and gas drilling and production, and more particularly to apparatus and methods for reliably transmitting information between downhole components.
For the past several decades, engineers have worked to develop apparatus and methods to effectively transmit information from components located downhole on oil and gas drilling strings to the ground's surface. Part of the difficulty comes from the fact that the operating environment for the transmission system can be extremely harsh, including temperatures as high as 200° C., pressures as high as 25,000 psi, and extremely abrasive and chemically corrosive conditions.
Another source of difficulty comes from the fact that a drill string is made up of hundreds of components, such as sections of drill pipe and various downhole tools. Thus, if the transmission system is integrated into each of these components, it must be capable of connections that can reliably transmit information across the joints between these hundreds of components.
Moreover, since these components are connected serially to create the drill string that may stretch for thousands of feet below the earth's surface, reliability is imperative. A failure in the transmission system of a single component can bring the whole system down and require an expensive “roundtrip” of the drill string to replace the defective component.
As an alternative to integrating a system into each of the drilling components, a transmission system has been developed known as mud pulse telemetry. Rather than using electrical connections, mud pulse telemetry transmits information in the form of pressure pulses through fluids circulating through a well bore. However, data rates of mud pulse telemetry are very slow compared to data rates needed to provide real-time data from downhole components. For example, mud pulse telemetry systems often operate at data rates less than 10 bits per second. At this rate, the low data resolution can prevent a driller from making decisions in real time. Since drilling equipment is often rented and very expensive, even slight mistakes can incur substantial expense. Part of the expense can be attributed to time-consuming operations that are required to retrieve downhole data or to verify low-resolution data transmitted to the surface by mud pulse telemetry. Often, drilling or other procedures are halted while data is gathered.
Since direct electrical connections between drill string components may be impractical and unreliable, converting electrical signals to magnetic fields for conversion back to electrical signals offers one solution for transmitting information between drill string components. One such system is disclosed in U.S. Pat. No. 6,670,880. These types of elements are referred to as inductive couplers. An inductive coupler functions by converting electrical signals to magnetic fields for transmission across the tool joint. A corresponding inductive coupler located on the next downhole component converts the magnetic field back to an electrical signal where it may be transmitted along the drill string.